A hypothesis-driven laboratory is described that introduces students to the complexities of ecosystem function. Students work with live algae, brine shrimp, and sea anemones to test hypotheses regarding the trophic interactions among species, the exchange of nutrients and gases, and the optimal ratio of producers to consumers and predators in enclosed, artificial-seawater ecosystems. In the initial iteration of the activity, multiple ecosystems that differ in their composition of organisms are assembled according to simple recipes by individual students or small lab groups. Two weeks later, the ecosystems are deconstructed, and the students tally the organisms and assess key water-quality measures, including salinity, pH, and ammonia levels. Some sample data and results are presented, as well as tips for successful implementation.